Distributed parameter models of blood-tissue exchange are increasingly used to interpret multiple tracer dilution data in regional kinetic studies. In all the studies where this modeling approach has been applied, parameter values are estimated from the data, and obtaining a measure of the precision with which the model parameters are estimated is crucial, since these parameters measure physiological processes. However, the standard approach to deriving precision of parameter estimates does not take into account the uncertainty of those model parameters that are fixed, typically, plasma flow and volume. Thus, the precision of parameter estimates is not realistic and, in all likelihood, is overestimated. The aim of this project is to envision methods to obtain a theoretically sound measure of the precision of parameter estimates, which takes into account both measurement error and the error associated with the fixed parameters.